The present invention relates to a device and method for optical sensing of substances or environmental conditions.
As one particular non-limiting application of the present invention, the invention will be exemplified below with reference to applications for sensing ammonia. Ammonia is an extremely important bulk chemical widely used in fertilizers, plastics and explosives, and also implemented as a coolant in large industrial refrigeration systems. On the other hand it is a toxic and flammable gas and therefore needs to be monitored. Ammonia is also listed as one of the marker molecules in breath that could be used to identify diseases like Uremia and kidney impairment.
Various sensors have been proposed for ammonia detection. Many take advantage of the basicity of ammonia by employing a pH sensitive dye. The principle of these sensors is based on the change of color of the detecting molecule immobilized in the optical structure in the presence of gas. Several approaches have been reported for realizing optical ammonia sensors based on pH indicators in fibers, waveguides, or immobilized into porous structures. The detection of ammonia in a humid environment like animal breath is of particular interest. In this reaction, salvation of gaseous ammonia and the pH indicator is required, because the protonation/deprotonation reaction is mediated by water. Moisture is therefore an important factor in this sensing mechanism, since it definitely influences the sensor's reading towards ammonia. In some cases, the response of the sensor towards water vapor may cause cross sensitivity with ammonia, because these two molecules have similar size and volume. Researchers have therefore recognized that water vapor has to be accurately monitored simultaneously with ammonia so these sensors could to be used in practical applications.
U.S. Pat. No. 6,897,965 to Ghadiri et al. discloses an approach for substance detection in which a layer of porous silicon (PSi) is impregnated with an indicator material of which the refractive index changes when it is exposed to the corresponding substance. The change in the refractive index of the layer is detected as a shift in the reflected interference pattern generated by the layer.
In “Biosensing Using Porous Silicon Double-Layer Interferometers. Reflective Interferometric Fourier Transform Spectroscopy” (Claudia Pacholski et al., J. Am. Chem. Soc., 2005, 127 (33), 11636-11645) and “Humidity-Compensating Sensor for Volatile Organic Compounds Using Stacked Porous Silicon Photonic Crystals” (Anne Ruminski et al., Adv. Funct. Mater. 2008, 18, 3418-3426), this approach is expanded to a two-layer structure in which variations in the optical properties of two stacked layers are sensed simultaneously to determine two different variable parameters. As detailed in Pacholski et al., the stacking of two sensing layers considerably complicates the spectral analysis of the output, since each layer individually and the combination of the two layers each generate a corresponding interference pattern in the reflected spectrum. While proposing a solution for the suggested two-layer structure, this approach does not seem to be suitable for generalization to more than two layers.
There is therefore a need for a device which would facilitate simultaneous sensing of a plurality of substances and/or environmental conditions in a fluid by straightforward spectral analysis, and which would facilitate scaling up of the device to sense multiple parameters.